An omnidirectional microphone (such as the ME 2) is good for general use when the source moves position around the microphone. An example would be a ME2 lavalier picking up a very animated person speaking who has a lot of head movement. Omnidirectional microphones have a spherical pickup pattern. With an omnidirectional microphone you do need to be aware of picking up unwanted sources and especially be careful of feedback (when live amplification is being used).
A cardioid microphone (like the ME 4) is a narrower pick up pattern. A cardioid microphone is good for situations when you need to reject external sound from the rear of the microphone and especially good for reducing the risk of feedback in live amplification environments.
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